I know that the books say it does not but I have always questioned that. I sometimes believe under high scrutiny that there is a slight amount of sa added to the optical chain. Has anyone actually tested a scope with a paracorr or paracorr 2 added to the system? I plan to do some null comparisons when I have a chance but am wondering if it has been done at all or do we just take TVs word for it???

Darren
Not owning one I have always wondered this as well, which - for me -is also related to the question of just how well corrected a catadioptric a paraboloid plus Paracorr is.

Now we already know that simpler coma correctors such as a Rosin introduce undercorrection, and so need to be paired with an appropriate Hyperboloid but where the net result is now excellent overall correction. But the Paracorr is touted as achieving this (as can a Wynne) with a paraboloid, the tradeoff with the Paracorr being a slight negative power (barlowing effect). But with respect to definitively settling your question, one problem is that I don't think the optical prescription for the Paracorr is out there. As such I don't know if any spot diagrams are available.

All that is really needed, I think, is a paraboloid of sufficient quality to star test with virtually identical intra and extra focal patterns. Such mirors while rare are most definitely out there. Star testing is so annoyingly sensitive to SA, that now introducing the paracorr into the light path of such a mirror wold clearly show SA well below any reasonable level of optical significance. I think such a simple test as this, under good seeing, would settle this question. (at least for that particular Paracorr! LOL)

I've done extensive star testing on an 8 inch f3.3 scope for the past several months with and without a Paracorr 2. I've not seen any difference in SA with and without the P2, but there is certainly a small difference among some of my eyepieces.

I've done extensive star testing on an 8 inch f3.3 scope for the past several months with and without a Paracorr 2. I've not seen any difference in SA with and without the P2, but there is certainly a small difference among some of my eyepieces.

Howard, JimGiven that star testing is generally cited as being sensitive to aberration at the wavefront of 1/20th wave or better, such a result - to the extent that it is generally true (Your P2 is not some fortuitous fluke, etc) - would seem to be definitive. While other testing methods (Double pass autocollimation? Yes/No?) might yield higher sensitivity, we would be well beyond the realm of any visual or imaging significance.

I would wonder if there is a difference between P2 and the original in this regard?Joe

Jim's comment is correct. The correction does not hit the zero point on axis, as coma would. Ergo, some SA is added, but it is less than the size of the Airy disc, which means invisible, and having no effect on resolution.Note that coma is corrected to smaller than the Airy disc down to f/3.5 in a field as wide as 40mm.

But with respect to definitively settling your question, one problem is that I don't think the optical prescription for the Paracorr is out there. As such I don't know if any spot diagrams are available.

SFAIK the amount of SA added solely by the Paracorr is tiny. Otherwise it would mean little to point out (as Al also did) the design SA of certain of the then-new Ethos line - as in around 1/100th wave.

What do you think of this test for the degree of spherical aberration introduced to an optical train by the Paracorr?

Test apparatus required:
Paracorr, a 250 line/inch Ronchi eyepiece, an accurately-collimated newtonian reflector which is known to be free of significant spherical aberration.

Method:
1. wait for a near-ideal night and observing conditions with respect to great seeing and minimal scintillation.

2. allow the system to achieve, as closely as possible, thermal equilibrium

3. critically examine (or even photograph) the images in the Ronchi eyepiece with and without the Paracorr inserted and look for signs of SA.

Would this test be sensitive enough to answer the intent of OP's query? Has anyone tried something like this before?
------
C

I've done the star test both with and without Paracorr and found no visible difference at all with an on-axis star of magnitude 2 (not as much seeing-related flare as a brighter star). No SA at all or any aberration I didn't already know was in the scope.

However, take that star off-axis, and the WITH Paracorr star is definitely better focused and has less flare.
Near the field edge, the star is still well-focused and without flare.
Without the Paracorr, only the dead center (or a tiny portion thereof) is sharp.

I suspect the amount of SA added or strehl reduced is dependent on f ratio. Maybe a star test doesn't show any noticeable differences at f5 but f4 could be another story. I think you would need more than a simple ronchi grating to reveal anything but rather a true null test. That would be the equivalent of taking one edge of a line on the ronchi test and expanding it to cover the whole surface. That gives you a much more sensitive null test and requires using either Polaris or having tracking to make work. I plan to try to a null test with and without a paracorr at f/4.4 here at some point soon.

Even thou the light is inside the Airy disk it is increased in size from the Paracorr so the Strehl ratio will be worse when using it with a perfect parabola. The other problem is that your assuming that your optics are dead perfect, if not then the SA from the Paracorr will add to the spherical aberration of the primary and can result in an image that is less then diffraction limited. The Paracorr adds undercorrected spherical to the image. This is why with the Type-I units, they were best teamed up with the primary that was over corrected to a mild hyberbola. If you use a single pass test using another telescope to produce a collimated beam, you need to remember that any error in that telescope directly adds to the test results. So if the optics have an 1/8 wave of under correction from the collimating telescope, that will adds to what ever the optics are in the telescope that has the Paracorr attached. So you can have a situtation were both telescopes are 1/8 wave under corrected and in addition to what the Paracorr is adding and when you test the system you see a large amount of undercorrection of over 1/4 wave. This would lead to false negative result that the Paracorr adds a fair amount of spherical. So just be careful that you understand the accuracy and precision of your test methods.

Dave,Very interesting. So the paracorr adds undercorrection. I wonder if any opticians who KNEW that the mirror was gonna be used with a SIPS in place has ever intentionally added a slight amount of overcorrection as a result.

Dave,
Very interesting. So the Paracorr adds undercorrection. I wonder if any opticians who KNEW that the mirror was gonna be used with a SIPS in place has ever intentionally added a slight amount of overcorrection as a result.

If it adds, say, 1/100 wave of undercorrection, which mirror maker would make a mirror accurate enough to add 1/100 wave of overcorrection to compensate? None.
This is a little like debating whether a 9-point mirror support is poorer than an 18 point if the 9-point deviates from perfect by 1/250 wave and the 18-point by 1/500 wave. Neither will produce anything visible.
Look at the charts. The amount of axial SA is smaller than the Airy disc all the way to below f/3. And look at the miniscule amount added. It won't be visible at all. If you see SA when star testing with the Paracorr in place, it's due to the mirror.

I would like to see bench testing done with and without a paracorr in place for a relatively fast mirror. 1/100 wave difference is the same as no effect and I don't believe this is the case with faster mirrors. The Telescopes Eyepieces Astrographs book doesn't get real specific on this. It doesn't even mention the paracorr by name and says it has a mag of 1.11X instead of 1.15. There is also no detailed spot diagram I suspect since the specs aren't avaialble.

The Paracorr adds more then 1/100 of wave error since the top of yellow shaded area on the Televue graphs is the diffraction limited cut off of 1/4 wave and the y-axis is linear. So for each graph one can divide that area up and determine what the scale would be in fractions of a wave. So the Paracorr is adding about 1/10 of wave or more of SA depending on what the f-artio of the mirror it is used on. For example if you look at the curve for the F/3.5 mirror the diffraction limited area starts at just a little under 3 on the graph so that is 1/4 wave and the Paracorr data starts at about 1.3 or so, which would translate into roughly an 1/8 wave of under correction. http://www.televue.c...?id=61&Tab=_con
I believe it has been discussed here, a number of years ago, of designing an astrograph using at that time the Paracorr which today is the Type-1 version in which the primary was made hyberbolic to correct for the under correction introduce by the Paracorr. I thought that Televue might have sugguested this as well.
The older Type-1 unit added more undercorrection then the new Type-2 units.

Very interesting. So the paracorr adds undercorrection. I wonder if any opticians who KNEW that the mirror was gonna be used with a SIPS in place has ever intentionally added a slight amount of overcorrection as a result.

You might just discuss the amount of undercorrection the original Paracorr introduced with Al himself. That idea was on the table but I passed. However anybody else who wanted to probably could. More interesting however is to build hyperboloidal mirrors to a conic designed into a Paracorr III that would also get flatter field...

Dave is correct here regarding type I vs type II, and indeed it adds more for faster mirrors, basically the price you pay for the coma reduction.

NB: 1/100th wave was re. the Ethos on-axis SA design amount, not the Paracorr I or II and not the combination of the two. In practice (of course) the optics aren't manufactured at design levels, SFAIK, but the newer designs are intended to function better with the coma correction at fast f/ratios than older ones could. A lot of it has to do with how the converging beam intersects the elements of the EP as it approaches the focal plane, and it ain't simple.

The Paracorr adds more then 1/100 of wave error since the top of yellow shaded area on the Televue graphs is the diffraction limited cut off of 1/4 wave and the y-axis is linear. So for each graph one can divide that area up and determine what the scale would be in fractions of a wave. So the Paracorr is adding about 1/10 of wave or more of SA depending on what the f-artio of the mirror it is used on. For example if you look at the curve for the F/3.5 mirror the diffraction limited area starts at just a little under 3 on the graph so that is 1/4 wave and the Paracorr data starts at about 1.3 or so, which would translate into roughly an 1/8 wave of under correction. http://www.televue.c...?id=61&Tab=_con
I believe it has been discussed here, a number of years ago, of designing an astrograph using at that time the Paracorr which today is the Type-1 version in which the primary was made hyberbolic to correct for the under correction introduce by the Paracorr. I thought that Televue might have sugguested this as well.
The older Type-1 unit added more undercorrection then the new Type-2 units.

- Dave

The yellow shaded area is the size of the Airy disk, not 1/4 wave of aberration.

When the image is larger then the Airy disk it is no longer diffraction limited by definition. So when one has exactly 1/4 wave optics they produce an image in which all the light just fits inside the Airy disk. So this is the upper bounds of the shaded yellow area. So one can relate the theoretical diameter of the spot formed when using a Paracorr which in this case is labeled Airy disk radius to the wave front and estimate the amount of spherical aberration the Paracorr is generating.